This invention relates generally to automotive or the like headlamps, especially of the halogen type, which include a lamp having both "low" and "high" beam filaments and more particularly to a wiring scheme therefor.
With the change from a foot-operated, floor button switch to a hand operated, steering column lever, switch for operating the high beams in American-built automotive vehicle headlamp systems, a "flash to pass" feature was also easily incorporated therein. The "flash to pass" feature, which has been provided in European and Japanese made automotive vehicles for some time, enables the driver to momentarily operate the headlamp high beam while continuing to operate the headlamp low beam. This "flashing" of the high beam is used primarily to indicate to a driver ahead that one wishes to pass.
It has been found that under certain conditions in commercially available dual beam headlamps, especially those of the halogen type, operation of the headlamps in a "flash to pass" mode can result in the production of an electric arc within the lamp envelope between the high and low beam filaments, which in turn can cause destruction of the low beam filament.
The dual beam headlamps in which the latter has been found to occur are constructed so that the high and low beam lamp filaments are closely adjacent and overlap each other with the central section of the high beam filament normally being aligned with the positive terminal of the low beam filament. The lamp is wired so that the same physical ends of both the high and low beam filaments are connected electrically to the negative terminal of a conventional 12-volt battery of the automotive vehicle.
It has been found that in headlamps of the type described, if the low beam lamp filament has been energized for more than 0.25 seconds but less than 1 minute and the "flash to pass" lever is operated in a rather rapid motion, arcing between filaments is likely to occur with possible damage to the low beam lamp filament resulting.
It has been discovered that the time between 0.25 seconds and 1 minute after the energization of the low beam filament of the lamp is critical because of the decreased pressure of the gas within the lamp envelope. At this time, the gas in the lamp envelope is at a pressure which is especially conducive to the establishment of electrical arcs therethrough.